SFO Director David Green announced the decision to “accept the Libor matter for investigation” in an e-mailed statement today.

Britain’s second-biggest bank was fined $451 million last week in the U.K. and U.S. after it admitted to submitting false Libor rates to benefit derivatives trades and bolster its own financial position. Chancellor of the Exchequer George Osborne told lawmakers last week that the FSA was in touch with prosecutors at the Serious Fraud Office about a possible criminal investigation. Opposition Labour leader Ed Miliband and London Mayor Boris Johnson both called for criminal probes.

Richard Alderman, who stepped down as director of the SFO two months ago, had said the agency wasn’t involved in the inquiry. People familiar with the FSA probe said last month that individuals in the case were unlikely to face a criminal investigation in the U.K. and that suspects were told to expect civil penalties. The U.S. Department of Justice is already conducting a criminal probe.

Citigroup Inc., Royal Bank of Scotland Group Plc, UBS AG, ICAP Plc, Lloyds Banking Group Plc and Deutsche Bank AG are among the firms some regulators are investigating. A total of 18 banks are surveyed as part of the process of determining Libor and related rates.